Defects in synaptic transmission in the auditory brainstem or cortex can give rise to central auditory processing disorders where the brain cannot correctly process sound. Investigating the synaptic mechanisms governing the processes of hearing in the auditory brainstem will allow for a better understanding of the etiology of human hearing disorders. The long-term objectives of the proposed research are to define the mechanisms that govern short-term plasticity at the auditory brainstem synapse called the calyx of Held, and to determine the functional importance of plasticity at this synapse for sound localization. The calyx of Held is a giant glutamatergic synapse in the auditory brainstem that functions as a fast excitatory synapse for sound localization. The calyx displays a form of short-term plasticity called post-tetanic potentiation (PTP), which can be induced by high-frequency (tetanic) stimulation and is dependent on elevations in presynaptic residual Ca2+ concentrations. Specifically, increases in presynaptic Ca2+ activate Ca2+-dependent isoforms of protein kinase C (PKCCa). While the calyx has long been studied as a model synapse, the physiological significance of PTP at the calyx has not been explored. Calyces are glutamatgeric synapses that project onto principal neurons of the medial nucleus of the trapezoid body (MNTB). The MNTB ?LSO synapse has been recently discovered to co-release glutamate/GABA/glycine during the period of synaptic refinement prior to hearing onset. Calyceal PTP is hypothesized to modulate the fidelity and temporal precision of MNTB firing remains unknown, and to increase release probability at the MNTB?LSO synapse to favor glutamatergic transmission. Thus, forms of short-term plasticity like PTP could play a major role in shaping the sound localization brainstem circuit. The specific aims of this research proposal are to (1) determine the dynamics of PTP under physiological conditions at the calyx of Held, (2) investigate the functional role of calyceal PTP in the brainstem sound localization pathway, and (3) dissect the PTP molecular cascade to selectively alter short-term plasticity at the calyx. The primary techniques used to achieve these aims include whole-cell electrophysiology recordings, bulk presynaptic Ca2+ loading in brain slices, and two-photon imaging of Ca2+ transients. The completion of this proposal will provide considerable insight into the functional roles of PTP at a key synapse in the auditory brainstem. The expertise of Dr. Wade Regehr and the environment of Harvard Medical School provide me with extensive training in electrophysiology and imaging techniques necessary to complete my goals.